All-Optical Signal processing using Highly Nonlinear Photonic Crystal Fiber

The use of HNL-PCF in optical communication systems has been investigated in this thesis. The investigation has been done with respect to the future of telecommunications in an all-optical system. The PCFs used have all been used for all-optical signal processing as part of an optical component. A large part of the work performed for this thesis has been on supercontinuum generation in a HNL-PCF and the use of such a supercontinuum in a system experiment. It has been shown how a supercontinuum generated by a HNL-PCF could form the basis for a multi wavelength pulse source in a WDM system. The thinking behind using a supercontinuum in an optical system is the same whether the fiber is a HNLF or a HNL-PCF, but the extra possibilities given by the freedom of design of the PCF technology appeals to using a HNL-PCF for a WDM source. The possibility of single-mode guidance at all wavelengths and the possibility of large differences between the refractive indices of the core and the cladding by using air-holes, makes PCFs suited for custom made components. By testing a HNL-PCF as a medium for supercontinuum generation at various dispersion values and at the same time using that supercontinuum in an optical system showed the importance of the fiber design. Different HNL-PCFs were tested for supercontinuum generation with the widest spectrum seen to be 210 nm (20 dB bandwidth). A comparison between two supercontinua generated by two different HNL PCFs with different dispersion profiles, but similar nonlinear coefficient, showed the dependence of supercontinuum on the dispersion in the HNL PCF. It is not only the generation of a supercontinuum that is dependent on the dispersion in the fiber, but also the slicing of the supercontinuum. An experiment using supercontinuum covering both the C- and L band and having a 20 dB bandwidth of -70 nm was discussed. A comparison between different pulse sources also showed the importance of low timing and amplitude jitter in the pulse source. The strict requirements on the pulse source and the fiber design in order to get a broad supercontinuum limits the choice of modulation format of the signal. The modulation format is also dependent on transmission in the optical system and dependent on the pulse source used to generate the supercontinuum. It is believed that by satisfying strict demands on the pulse sources and the fiber design, could the use of a supercontinuum in a commercial optical system become a possibility in the near future. In this thesis the supercontinua have all been generated using a 10 GHz pulse train, but higher repetition rates such as 40 or 160 GHz are also possible due to the speed of the Kerr-effect [48]. For bit rates higher than 40 Gbit/s all-optical regeneration is the only possible way of regenerating a signal with the current technology. Transforming the current telecommunication network into an all-optical network will require an all-optical regeneration of the optical signal. At the current time (May 2005) all-optical regeneration is a tool only used in laboratory experiments for regeneration of signals with bit rates of 80 and 160 Gbit/s or higher. One method of 3R-regeneration was discussed in this thesis. The well known NOLM configuration was used as a part of the setup in an attempt of achieving regeneration of an optical signal. More successful 3R-regeneration experiments have been reported [40] for high speed signals. An extensive investigation into using HNL-PCF as part of an alloptical wavelength converter has been performed in chapter 5. Especially FWM has been used in order to perform wavelength conversion since FWM is a transparent process. Using a HNL-PCF with a flattened dispersion profile, a 40 Gbit/s RZ-DPSK signal was wavelength converted over 31 nm. The same fiber was also used when showing the wavelength conversion of a 6×40 Gbit/s DPSK signal. The conversion efficiency was found to be better than −21 dB in both cases. Using a HNL PCF as an integral part of a transparent all-optical wavelength converter shows one of the many possible applications for a HNL-PCF in optical systems. A combination of a HNL-PCF and FWM generates a very powerful tool for all-optical wavelength conversion. The transparency of an optical system is tested by mixing different modulation formats. By using 40 Gbit/s equipment and combining two different modulation formats a signal rate of 80 Gbit/s is achieved. The 80 Gbit/s RZ DPSK-ASK signal is wavelength converted in a HNL-PCF using FWM. With a conversion efficiency better than −19 dB and a wavelength conversion over -10 nm a simple but efficient all-optical wavelength converter was realized. The dispersion profile of a fiber remains one of the most important parameters to control for the fiber designer. The manufacturing of a HNLPCF with negative dispersion slope and zero dispersion wavelength in the C-band made it possible to build an OPC using this fiber. Optical phase conjugation is one way of dispersion compensating a fiber span. The experiment performed in chapter 5 shows that, by using optical phase conjugation in a HNL-PCF, a successful transmission over a PCF fiber link is made possible. Without the optical phase conjugation component, transmission of a 40 Gbit/s signal would not have been possible due to dispersion in the fiber link. Two methods of characterizing a fiber are also presented. An interferometric dispersion measurement technique and a technique for measuring the nonlinear coefficient of a fiber are used to obtain measurement results from various PCFs. The dispersion of a PCF was measured over 1100 nm and a zero dispersion wavelength at 1064 nm was found. The results obtained in the work presented in this thesis show that PCF technology and especially HNL-PCF can be used in an all-optical system for signal processing.